RewriteEngine On

RewriteRule ^thank-you/?$ thank-you.php [L]

RewriteRule ^sitemap\.xml$ sitemap.php [L]

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([a-zA-Z0-9-]+)/?$ profile.php?username=$1 [L,QSA]